Recently, there has been an increasing interest in energy storage technology. Batteries have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, the research and development into a novel electrode and a novel battery that can improve capacity density and specific energy have been made intensively in the field of the secondary batteries.
Among currently used secondary batteries, lithium secondary batteries developed in early 1990's have a higher drive voltage and a much higher energy density than those of conventional batteries using an aqueous electrolyte solution (such as Ni-MH batteries, Ni—Cd batteries, H2SO4—Pb batteries, etc). For these reasons, the lithium secondary batteries have been advantageously used. However, such a lithium secondary battery has disadvantages in that organic electrolytes used therein may cause safety-related problems resulting in ignition and explosion of the batteries and that processes for manufacturing such a battery are complicated.
Recently, lithium-ion polymer batteries have been considered as one of the next-generation batteries since the above disadvantages of the lithium ion batteries were solved. However, the lithium-ion polymer batteries have a relatively lower battery capacity than those of the lithium ion batteries and an insufficient discharging capacity in low temperature, and therefore these disadvantages of the lithium-ion polymer batteries remain to be urgently solved.
Such a battery has been produced from many companies, and the battery stability has different phases in the lithium-ion polymer batteries. Accordingly, it is important to evaluate and ensure the stability of the lithium-ion polymer batteries. First of all, it should be considered that errors in operation of the batteries should not cause damage to users. For this purpose, the Safety and Regulation strictly regulate the ignition and the explosion in the batteries.
In order to solve the above battery safety-related problem, there has been proposed an organic/inorganic composite separator having a porous active layer formed by coating at least one surface of a porous substrate having pores with a mixture of inorganic particles and a binder polymer. The porous active layer formed of this conventional organic/inorganic composite separator shows homogeneous composition morphology toward a thickness direction, as shown in FIG. 2B and FIG. 3B. However, if the electrochemical device is assembled with the organic/inorganic composite separator, it has disadvantages in that inorganic particles in the porous active layer are detached and a lamination characteristic toward electrodes is deteriorated during a winding process, etc. If a content of a binder polymer in the porous active layer is increased so as to solve the above disadvantages, characteristics such as the peeling and scratch resistances, the lamination characteristic toward electrodes, etc. in the assembly process of the electrochemical device may be rather improved, but porosities in the porous active layer are decreased since the inorganic particles are present in a relatively lower content, resulting in deterioration in performances of the electrochemical device, and the safety of the separator is also reduced due to the introduction of the porous active layer.